Seismic refraction: imaging the lower crust and upper mantle: PART 1

Throughout the cruise we’ve been deploying ocean-bottom seismographs (OBSs) onto the seabed.  We have now finished a total of 81 deployments and, to date, we have successfully recovered 48 of those, each with its own multi-component dataset from three geophone sensors used to measure three-dimensional ground motion, and a hydrophone which measures pressure waves in the water column.

 

An OBS being deployed into the ocean. Each instrument has a variety of different parts: the four sensors to make the measurements, the data logger to record these and keep time, an acoustic release we signal from the ship to bring the OBS back to the surface, an anchor and float to sink and raise the instrument, a flag, light and radio to enable location of the OBS once on the surface, and a strayline to help recovering the OBS back onto the ship.

Twenty-seven OBSs are currently awaiting recovery from the seabed once the final phase of airgun shooting is complete, together with a further four that were deployed on the Sandra Ridge to the north of the work area to record local earthquakes, that will be recovered during our transit to Balboa for the end of cruise.

So why do we record airgun seismic signals using seabed instruments?

The multi-channel streamer towed behind the vessel measures signals that travel near-vertically down into the sub-seabed and reflect from the boundaries between individual rocks layers due to their difference in density. The resulting images are in two-way travel time of the recorded reflections, and give a cross-sectional-like view of the sub-surface but contain no information that allows them to be converted into true depth, so we cannot answer the question “how deep is this layer beneath the seabed?” or “how thick are these sediments?”

To answer these questions we need to know the speed, or velocity, at which each seismic signal travels through each layer, including the water layer. The water layer is a relatively easy velocity to measure using a sound velocity tool suspended from a wire lowered to near the seabed and back again. The velocities of rock layers are not so easy to measure. However, with these velocities we can convert the measured reflection times into distance much as you would use the speed limits on roads and the distances between two points to work out the time it would take to travel between A and B.

Diagram of marine seismic acquisition, showing the acoustic source (airguns), and the two types of receivers we are using: the multi-channel streamer towed behind the ship, and the OBSs on the seafloor.

 This is where an ocean-bottom seismograph (or 35 of them- which is the maximum we have had deployed along any seismic line during this cruise at any one time) comes in handy and we use the seismic refraction approach. By synchronizing their internal clocks with the same clock used to time the airgun shots (our acoustic pulses), we can measure the time it takes for signals to travel from the airgun array to each OBS on the seabed, and if we know their distance away from the shots we can work out the speed the signals travel through each sub-surface layer. We use GPS for this purpose as it can equally well provide an accurate time source as it can tell you where you are at any point.

The figure above shows how the method works and how it can be used in conjunction with reflection surveying killing two birds with one stone and making cost-effective use of the expensive ship time that we have been awarded for this project.

**Tune back in tomorrow for Part 2 of this post!**